AMD APU Undervolting: Reduce Power Consumption by 32%!

We recently received a review sample of AMD’s recently introduced A-series APU A8-3850. While we aren’t finished with our testing, we wanted to check just how far AMD went to make sure the glueing of K10.6 CPU would go with the world’s first SOI-based GPU. Instead of overclocking, we ventured the other way and came to quite remarkable results with undervolting the APU.

You have to know that the default voltage at 2.9GHz is 1.424V, which is quite a lot considering that Llano is manufactured on a 32nm SOI process. For comparison, Intel’s 32nm Core i3-2100 pulls in 1.1 Volts. When put under full load using Prime95 "In-place large FFTs" stress test, the power consumption rises to 184W. When additionally loaded with a GPU burn-in test (we used MSI Kombustor test), the power consumption tops out at 209W for the whole system. This is up from a rather impressive 59W when running idle on the Windows desktop. In idle mode the CPU cores clock down to 800MHz at 1.024V though.

Undervolting from 1.42 to 1.13V resulted in significant decrease of power consumption

We toyed around a bit and finally achieved a stable -0.275V undervolt. When we set the voltage offset to -0.3V the system crashed with a blue screen. With our stable setting though, it ran the CPU and GPU stress tests without a hassle. With only the CPU cores loaded power consumption was down to 123W. Adding GPU load to that raised it to about 142W. As you can see on the CPU-Z screenshot, with this setting, the CPU cores were operated with only 1.136V (note that due to how VIDs work, this is actually -0.288V). Running idle, power drain dropped to 55.6W, which is only a minor improvement, since most savings come from power gating in this state.

However, full load going from 209 to only 142 Watts for the whole system is something that sparks our interest – if you can keep the system running at this power level, you’re saving 32% of your annual computer power bill, which is not a small change regardless of the country you live in. We feel that AMD left plenty of headroom for overclocking, but actually it turns Fusion into a majestic power saver.

From 209W to just 141.8 Watts by simple undervolting operation

All power figures presented here are measured at the wall outlet for the complete system. The mainboard we employed is a Gigabyte A75M-UD2H µATX board running BIOS F3b. For reviewing purposes we ran a minimal system configuration with only one HDD, USB keyboard and mouse attached to the system. The system was powered by a 600W Silverstone Strider ST60F power supply, which is rated at 80-85% efficiency at 20-100% load. It might not be the optimal choice for such a low power system, so a weaker power supply with higher efficiency at those levels might lower consumption by a few more watts.

You might have noticed that we used an seven year old Thermaltake K8 Silent Boost cooler, originally designed for first Athlon 64 processors. We deliberately used the oldest AMD heatsink we could find to demonstrate just how upgradeable AMD’s ecosystem is. Even though the company changed seven processor sockets (754 to 939, AM2, AM2+, AM3 and now AM3r2 and FM1 for Fusion), you can use the identical heatsink at default clock.

We will conduct further tests with the system and are planning to deliver a full review in the near future. If you have ideas for interesting tests we might not have thought about, feel free to tell us in the comments below.